The present invention relates to multilayer coextruded light-reflecting films which have a narrow reflection band because of light interference. When the reflection band occurs within the range of visible wavelength, the film is iridescent. Similarly, when the reflection band falls outside the range of visible wavelength, the film is either ultraviolet or infrared reflecting.
The multilayer films and methods by which they can be produced are known in the art. In this connection, the reader's attention is directed to the following U.S. patents which are hereby incorporated by reference: U.S. Pat. Nos. 3,328,003; 3,442,755; 3,448,183; 3,479,425; 3,480,502; 3,487,505; 3,511,903; 3,549,405; 3,555,128; 3,565,985; 3,576,707; 3,642,612; 3,711,176; 3,759,647; 3,773,882; and 3,801,429.
The multilayer films are composed of a plurality of generally parallel layers of transparent thermoplastic resinous material in which the contiguous adjacent layers are of diverse resinous material whose index of refraction differs by at least about 0.03. The film contains at least 10 layers and more usually at least 35 layers and, preferably, at least about 70 layers.
The individual layers of the film are very thin, usually in the range of about 30 to 500 nm, preferably about 50-400 nm, which causes constructive interference in light waves reflected from the many interfaces. Depending on the layer thickness and the refractive index of the polymers, one dominant wavelength band is reflected and the remaining light is transmitted through the film. The reflected wavelength is proportional to the sum of the optical thicknesses of a pair of layers. The reflected wavelength can be calculated by the formula ##EQU1## In this formula, .lambda. is the reflected wavelength, M is the order of reflection, t is the layer thickness, n is the refractive index, and 1 and 2 indicate the polymer of the first layer and the polymer of the second layer, respectively. The quantity nt is the optical thickness of a layer. For first order reflection, i.e. when M is 1, visible light is reflected when the sum of optical thicknesses falls between about 200 and 350 nm. When the sum is lower than about 200, the reflection is in the ultraviolet region of spectrum and when the sum is greater than about 350 nm, the reflection is in the infrared region.
The quantity of the reflected light (reflectance) and the color intensity depend on the difference between the two refractive indexes, on the ratio of optical thicknesses of the layers, on the number of layers and on the uniformity of the thicknesses. If the refractive indexes are the same, there is no reflection at all from the interfaces between the layers. In the multilayer films, the refractive indexes of contiguous adjacent layers differ by at least 0.03 and preferably by at least 0.06 or more. For first order reflections, reflectance is highest when the optical thicknesses of the layers are equal although suitably high reflectances can be achieved when the ratio of the two optical thicknesses falls between 5:95 and 95:5. Distinctly colored reflections are obtained with as few as 10 layers; however, for maximum color intensity it is desired to have between 35 and 1000 or even more layers. High color intensity is associated with a reflection band which is reltively narrow and which has high reflectance at its peak. It should be recognized that although the term "color intensity" has been used here for convenience, the same considerations apply to the invisible reflection in the ultraviolet and infrared ranges.
The multilayer films can be made by a chill roll casting technique using a conventional single manifold flat film die in combination with a feedblock which collects the melts from each of two or more extruders and arranges them into the desired layer pattern. Feedblocks are described in the aforementioned U.S. Pat. Nos. 3,565,985 and 3,773,882. The feedblocks can be used to form alternating layers of either two components (i.e. ABAB . . . ); three components (e.g. ABCABCA . . . or ACBCACBC . . . ); or more. The very narrow multilayer stream flows through a single manifold flat film die where the layers are simultaneously spread to the width of the die and thinned to the final die exit thickness. The number of layers and their thickness distribution can be changed in inserting a different feedport module. Usually, the outermost layer or layers on each side of the sheet are thicker than the other layers. This thicker skin may consist of one of the components which makes up the optical core; may be a different polymer which is utilized to impart desirable mechanical, heat sealing, or other properties; or may be a combination of these.
The high refractive index component used heretofore in commercial production has been polystyrene (refractive index 1.60). Other high index resins which are optically suitable but which have disadvantages in terms of cost or difficulty of extrusion in the multilayer process are polycarbonate (1.59), vinylidene chloride (85%)-vinyl chloride (15%) copolymer (1.61), and polydichlorostyrene (1.62). Polystyrene in combination with such lower refractive index polymers as poly(methyl methacrylate), polypropylene, and ethylene vinyl acetate, all of which are close to 1.50 in refractive index, produces iridescent films of desirable optical properties which, however, reveal deficiencies in certain mechanical properties. For example, the adhesion between individual layers of the multilayer structure may be insufficient, and the film may suffer from internal delamination or separation of layers during use. The iridescent film is often adhered to paper or board for its decorative effect, and is then used for greeting cards, cartons, and the like. Delamination of the film is unsightly and may even lead to separation of the glued joints of carton. In addition, the solvent resistance and heat stability of such films are not as great as desired for widespread utilization.
Accordingly, it is the object of this invention to provide new and improved multilayer light-reflecting films which exhibit increased resistance to delamination, improved solvent resistance and/or improved heat stability. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.